Understanding the role of chromatin modifying factors in response to ionizing radiation (IR) will provide invaluable insights into questions of both how cancers start and how to cure cancers. The ATM (ataxia-telangiectasia mutated) protein is a major participant in .all cellular responses to IR. We have been studying the role of ATM in DNA damage repair, telomere chromatin structure and oncogenic transformation. Cells deficient in ATM have defects in DNA repair, display altered telomere chromatin structure and have a higher frequency of spontaneous as well as IR-induced oncogenic transformation. Recently, we identified a chromatin-modifying factor "hMOF", the human ortholog of the Drosophila MOF gene (Males absent On the First) which interacts with ATM. hMOF has histone acetyltransferase (HAT) activity. Cellular exposure to IR enhances hMOF-dependent acetylation of its target substrate, lysine 16 of histone H4 (H4- K16), independent of ATM function. Inactivation of hMOF results in abrogation of ATM function. Based on the facts that hMOF is involved in ATM function, expression of MOF fragment enhances oncogenic transformation in vitro and that tumors show loss of H4-K16 acetylation (H4-K16Ac), we hypothesize that hMOF is involved in tumorigenesis. In the proposed work, we will determine the link between MOF and tumorigenesis by the functional characterization of mouse Mof (mMof) in a murine cell system and pathobiology of mMof haploinsufficient and mMof conditional knockout mice. The in vitro studies will determine Atm dependent and independent mMof functions in response to IR for cell killing, DNA damage repair and oncogenic transformation. The impact of the loss of H4-K16Ac on the predisposition of mouse tumor development will be determined in mMof haploinsufficiency or mMof conditional knockout mice. Experiments described in this proposal will investigate the functional links among mMof, H4-K16Ac and spontaneous as well as IR-induced tumor formation. These studies will improve our understanding of the role of mMof in the IR response and tumorigenesis. Ultimately, understanding the basis for biochemical differences in chromatin structure between normal and tumor tissue could provide strategies for modifying the response to IR that could be useful in clinical radiation therapy.